The new turbocharged I-4 engines won't disappoint you. We have one in our Z4. 240hp is more than enough for an X3.
There are some standard equipment and option differences between the 28i and 35i. More comes standard on the 35i but most of the items can be added to the 28i.
The models and equipment (standard and optional) illustrated in this post reflect my misunderstanding of vehicles supplied by BMW AG to the German market. In other EU member states, the truthyness of my posts may vary. Please ignore this post. Subject to change.